Circuit arrangements for operating low pressure electric discharge lamps



Dec. 13, 1960 I. F. DAVIES ETAL 2,964,676 CIRCUIT ARRANGEMENTS FOR OPERATING LOW PRESSURE ELECTRIC DISCHARGE LAMPS Filed Aug. 15. 1958 AL ZQQLL 2,964,676 CIRCUIT ARRANGEMENTS FOR OPERATING LOW PRESSURE ELECIREC DISCHARGE LAMPS Ian Frederick Davies, Highgate, London, and Joseph Colin Vickery, Exeter, England, assignors to The General Electric Company Limited, London, England Filed Aug. 15, 1958, Ser. No. 755,258 Claims priority, application Great Britain Aug. 29, 1957 Claims. (Cl. 315-98) This invention relates to circuit arrangements for operating low pressure electric discharge lamps.

It is an object of the invention to provide a circuit arrangement for operating a low pressure electric discharge lamp from a direct current source, which may be of relatively low voltage.

According to the invention, a circuit arrang:ment for operating a low pressure electric discharge lamp from a direct current source comprises an oscillator circuit arranged to be energised by the direct current source and to supply oscillatory power to the lamp, the oscillator circuit including: a transistor amplifier; an output transformer having a ferromagnetic core on which are wound primary and secondary windings, the primary winding being connected to the output of the amplifier and the secondary winding being connected in a resonant circuit in which the discnarge path of the lamp is arranged to be connected; and means for applying to the input of the amplifier regenerative feedback which is derived via the output transformer.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing, which is a circuit diagram of an arrangement for operating a low pressure mercury vapour fluorescent electric discharge lamp of the type having electrodes which are adapted to be heated by power supplied from an external source.

Referring to the drawing, the arrangement comprises an oscillator circuit in which the active element is constituted by a matched pair of germanium P-N-P junction transistors 1 and 2, arranged to operate as a push-pull amplifier. The emitter electrodes of the transistors 1 and 2 are directly connected together and between the collector electrodes of the transistors 1 and 2 is connected the primary winding 3 of an output transformer 4 which has a ferromagnetic core; the primary winding 3 has a centre tapping which is connected to the negative terminal of a battery 5, the positive terminal of which is connectable via a switch 6 to the emitter electrodes of the transistors 1 and 2. The transformer 4 has a main secondary winding 7, which has several times the number of turns of the primary winding 3; the winding 7 is con nected in a resonant circuit which also includes a tuning capacitor 8 connected across the winding 7, a choke 9 having a ferromagnetic core, and the discharge path of the lamp 10, the choke 9 and the discharge path of the lamp 10 being connected in series across the winding 7.

The transformer 4 also has two auxiliary secondary windings 11 and 12 which are respectively connected so as to supply heating power to the electrodes 13 and 14 of the lamp 10. The regenerative feedback signal for maintaining the circuit in oscillation is derived by means of a feedback winding 15 on the transformer 4, the winding 15 being connected between the base electrode of the transistors 1 and 2 and having a centre tapping which is connected via the parallel combination of a resistor 16 and a capacitor 17 to the emitter electrodes of the transistors 1 and 2. The windings 3, 7, 11, 12, and 15 of the transformer 4 are wound in the senses indicated by the dots in the circuit diagram.

, In operation the rectifying action of the emitter junctions of the transistors 1 and 2 causes a standing voltage 2,964,676 Patented Dec. 13, 1960 ice to be developed across the capacitor 17, this voltage biasing the base electrodes of the transistors 1 and 2 positively with respect to their emitter electrodes, so that the amplifier normally operates under Class C conditions. Initiation of the oscillation when the switch 6 is closed is assisted by the provision of a resistor 18 connected between the negative terminal of the battery 5 and the base electrode of the transistor 2, the current supplied through the resistor 18 to the base electrode of the transistor 2 urging the transistor 2 into the conducting condition.

In the starting condition of the circuit arrangement, that is to say with the switch 6 closed but before a discharge is struck in the lamp 10, the oscillator circuit is relatively lightly loaded and the output waveform is therefore markedly non-sinusoidal. Thus, in each half cycle of the oscillation during which the transistor 1 conducts and tne transistor 2 is cut off, the transistor 1 is rapidly driven into a saturated condition, after which the base current of the transistor 1 remains substantially constant but the collector current in the transistor 1 increases so as to increase the magnetisation of the core of the transformer 4- and thereby sustain the voltage across the pri mary winding 3; at some point during the rise of the collector current in the transistor 1 the core of the transformer 4 will saturate, the collector current thereafter rising rapidly until it reaches a limiting value equal to L16 product of the base current in the transistor 1 and t.;e base-to-collecto-r current gain factor of the transistor 1. No further increase of the collector current is then possible so that the voltage across the primary winding 3 collapses rapidly, thereby causing the transistor 1 to be cut off and the transistor 2 to be rendered conducting so that the next half cycle of the oscillation is initiated. This next half cycle is similar in nature to that just described, but with the functions of the transistors 1 and reversed.

In this condition of the oscillator circuit, therefore, the output waveform is substantially rectangular, the rectangular wave having an amplitude at the primary winding 3 approximately equal to the voltage of the battery 5 and having a frequency which is largely determined by the time taken for the core of the transformer 4 to saturate during each half cycle after the relevant transistor 1 or 2 is saturated. A certain amount of overshoot occurs, however, on the leading edge of each half cycle of the waveform, due to ringing of the transformer 4, so that the amplitude of the voltage appearing across the lamp 10 is somewhat greater than would be the case if the waveform were perfectly rectangular, this assisting in starting the discharge in the lamp 10. The amount of overshoot depends on the coupling coefiicient between the two halves of the primary winding 3, and should not be allowed to be so great as to give rise to a risk of electrical breakdown of the collector junctions of the transistors 1 and 2 due to the occurrence of an excessive peak collector voltage when either transistor is in the cut off condition. For this reason, it will normally be desirable to utilize a bifilar winding for the primary winding 3 in order to ensure a high value for the coupling coeflicient between the two halves of the Winding 3.

In the running condition of the circuit arrangement, the loading of the oscillator circuit is greatly increased by the passage of the discharge current through the lamp 10, and the circuit is designed so that in this condition each transistor 1 or 2 is driven almost, but not quite, into the saturated condition during the half cycle of the oscillation in which it conducts; this may suitably be achieved by choice of the value of the resistor 16, which determines the value of the feedback current, having regard to the base-to-collector current gain factors of the transistors 1 and 2.

-matically correcting the load power factor.

In this condition, the core of the transformer 4 is never saturated, and the oscillator circuit therefore operates at a frequency approximately equal to the resonant frequency of the main secondary circuit, thus auto- The waveform of the voltage appearing across the secondary winding 7 is approximately sinusoidal so that by suitable design of the choke 9 the waveform of the discharge current in the lamp 10 may be made substantially sinusoidal, as is desirable from the point of view of obtaining a relatively long operating life for the lamp 10. The amplitude of the voltage appearing across the primary winding 3 is slightly less than the voltage of the battery 5, so that the amplitude of the voltage applied to the lamp 10 is less in the running condition of the circuit arrangement than in the starting condition; this enables a smaller choke 9 to be used than would be the case if these two amplitudes were equal.

If the voltage of the battery should drop, the load regulation of the circuit arrangement is relatively good, for the following reason. A drop in the voltage of the battery 5 causes the transistors 1 and 2 to be saturated during part of the half cycles during which they respectively conduct, with resultant squaring of the output waveform. The R.M.S. value of the output voltage is therefore greater than would be the case for a voltage of sinusoidal Waveform having the same amplitude, thus counteracting the decrease of the amplitude of the output voltage due to the drop in the voltage of the battery 5.

In one circuit arrangement, as described above, which has been used in practice the lamp is a standard tubular lamp of 40 watts rating, 1.2 metres long and 3.8 centimetres in diameter, and the transistors 1 and 2 are of the type GET 9 supplied by The General E ectric Co. Ltd., or of the type OCl6 supplied by Mullard Ltd. The transformer 4 is of the shell type, having a core formed from Radiometal" laminations 0.1 millimetre thick stacked to a depth of 9.6 millimetres, the central limb and windows of the core each having a Width of 9.6 millimetres, the outer limbs of the core each having a width of 6.4 millimetres, and the overall length of the core in the direction of the limbs being 3.5 centimetres; the windings of the transformer 4 are as follows, being wound in the order stated:

Primary Winding 367+67 turns of enamelled copper Wire of diameter 0.6 millimetre (bifilar wound).

Feedback winding -9+9 turns of enamelled copper wire of diameter 0.2 millimetre.

Auxiliary secondard winding 11-24 turns of enamelled copper wire of diameter 0.38 millimetre.

Main secondary winding 7--770 turns of enamelled copper Wire of diameter 0.2 millimetre.

Auxiliary secondary winding 1224 turns of enamelled copper Wire of diameter 0.38 millimetre.

The choke 9 is of the shell type, having a core formed from Silcor 2 laminations 0.35 millimetre thick stacked to a depth of 7.9 millimetres, the central limb of the core having a width of 1.1 centimetres, the windows of the core each having a width of 6.7 millimetres, the outer limbs 'of the core each having a width of 4.8 millimetres, and the overall length of the core in the direction of the limbs being 2.6 centimetres; this core is wound with 1,000 turns of enamelled copper wire of diameter 0.2 millimetre, and is provided with an air gap such that the choke 9 has an impedance of 1.38 kilohms to an alternating current of frequency 1.3 kilocycles/second and R.M.S. value 0.12 ampere. The remaining components of the circuit arrangement are as follows:

Battery 5-28 volts nominal. Capacitor 80.1 rnicrofarad, 350 volts rating. Resistor 16-25-45 ohms (precise value dependingon value of base-to-collector current gain factors of transistors 1 and 2), 0.5 watt rating.

Capacitor 178 microfarads, 12 volts rating.

Resistor 182.2 kilohms, 0.5 watt rating.

With these components the input power is approximately 24 watts, the power dissipated in the lamp 10 being approximately 16 watts. The oscillation frequency is approximately 1.3 kilocycles/second in the running condition and approximately 0.9 kilocycle/second in the starting condition. The arrangement is found to operate reliably even when the battery voltage is reduced to 21 volts.

In an alternative arrangement to that described above, the feedback signal, instead of being derived via a separate winding on the output transformer, might be derived from the main secondary circuit, for example by means of a further transformer.

In further alternative arrangements, the invention may be utilised in circuit arrangements for operating a plurality of discharge lamps. For example, the arrangement described above could be modified by adding a further lamp and choke connected in series across the winding 7, further auxiliary secondary windings being provided on the transformer 4 for supplying heating power to the electrodes of the further lamp.

We claim:

1. A circuit arrangement for operating a low pressure electric discharge lamp from a direct current source, comprising an oscillator circuit arranged to be energised by the direct current source and to supply oscillatory power to the lamp, the oscillator circuit including: a transistor amplifier; an output transformer having a ferromagnetic core on which are wound primary and secondary windings, the primary Winding being connected to the output of the amplifier and the secondary winding being connected in shunt with a capacitive impedance to form a resonant circuit therewith and in which the discharge path of the lamp is arranged to be connected; and means for applying to the input of the amplifier regenerative feedback which is derived via the output transformer.

2. A circuit arrangement according to claim 1, including means for biasing the amplifier so that it normally operates under Class C conditions.

3. A circuit arrangement according to claim 1 in which the magnitude of the feedback is such that in the normal running condition of the circuit arrangement the core of the transformer is not saturated at any point during the cycle of oscillation, whereas in the starting condition of the circuit arrangement the core of the transformer is saturated during part of the cycle of oscillation.

4. A circuit arrangement according to claim 1, in which the transistor amplifier includes a pair of like transistors arranged to operate in push-pull, the primary winding of the transformer being connected between corresponding electrodes of the two transistors.

5. A circuit arrangement according to claim 1, in which the discharge path of the lamp is arranged to be connected in series with a ballast impedance.

References Cited in the file of this patent UNITED STATES PATENTS 

